Prior art computer systems typically include a volatile memory for the storage and manipulation of information by an operating system and various software applications. When software applications behave in unexpected ways, they can cause the operating system to fail in catastrophic ways, referred to colloquially as a “system crash.” When a system crashes, there is no guarantee that the information stored in volatile memory can be salvaged. Typically, the user remedies the system crash by resetting the system. In the resulting boot cycle the operating system typically loses the ability to reference the information contained in the volatile memory or actually initializes the volatile memory, changing or destroying its contents.
Prior art solutions to this problem have taken various approaches. One approach requires a user manually to direct applications to save the contents of volatile memory to a non-volatile memory when significant amounts of information have been processed in volatile memory. An incremental improvement over this approach takes the form of modifications to the software applications themselves, whereupon they save the contents of volatile memory to a non-volatile memory when certain criteria are met. For example, the word processing program Microsoft Word™ from Microsoft Corporation, Redmond, Wash. has an option that automatically saves the contents of documents upon the elapse of a time period selected by the user.
These prior art systems have several failings. First, a failure in the operating system may prevent the functioning of any application-level safeguards. Second, safeguards that rely on regular human intervention are subject to human failings, such as when humans forget to invoke them. Third, safeguards that attempt to substitute application-administered criteria for human judgment and invocation fail in that they cannot guarantee that critical information would be saved when a human user would have chosen to save it.
A second set of prior art solutions to this problem has focused on hardware modifications to preserve the contents of volatile memory during a crash. Some prior art systems are arranged such that every read or write request to an operating system is simultaneously routed to a non-volatile memory. Such a system guarantees a record of memory contents that can be reconstructed during a boot cycle, but suffers from slowness during normal operation, because each transaction is conducted twice, and slowness during a boot cycle, because the operating system must locate the non-volatile record of transactions and reload them. Other prior art systems attempt the same techniques and suffer from the same problems, but reduce the magnitude of the delays by greater selectivity in the transactions actually recorded, or recording transactions in a way that is more amenable to reconstruction. Other prior art systems relying on hardware modification use non-volatile memories, such as electrically erasable programmable read-only memories (EEPROMs), Flash ROM, or battery-backed random-access memory. These systems have several drawbacks, including higher prices than normal volatile memories and the requirement of additional hardware. For example, Flash ROM often requires a charge pump to achieve the higher voltages needed to write to the memory, and suffers a shorter life than normal volatile RAM because of this process. Battery-backed RAMs rely on batteries that are subject to catastrophic failure or charge depletion.
A computer whose information is stored in a volatile memory resistant to loss or corruption resulting from system or application crashes would avoid the problems associated with the loss and recreation of data. The elimination of time-consuming data reconstruction would help make possible a fault-tolerant computer that offered continuous availability. The present invention provides those benefits.